Every D&D world needs a force of chaos to prevent the great nations of the world from simply conquering your setting and filling all the corners of the map. Exploration is a core tenet of Dungeons & Dragons, and chaos that keeps the forces of order in check make it possible for the heroes to explore the untamed wilderness and long-lost dungeons. Orcs are one way of keeping civilization from pushing too deep into uncharted territory, or at least forcing the settlements in that territory from developing into major cities.
Orcs are connected so closely to their pantheon that they are almost like a divine plague let loose upon the world. The Monster Manual even describes their clans as plague-like, as a tribe of creatures like bloodlust personified. . . . Orcs are a perfect way to introduce your characters to the power of gods beyond those that they worship.
The real question is why -- why does every civilization have similar myths? Why does every culture have legends of monstrous humanoids, and why are they are always depicted as fearsome and dangerous?Because the legends were real. The orcs were real.